Cloudflare: add a how-to regarding the use of `getCloudflareContext`

[dario-piotrowicz]

We do introduce getCloudflareContext (see bindings docs) and mention it a few times, however the utility has some nuances such as:

• it requires initOpenNextCloudflareForDev to be called
• it can't be called in static routes
• it relies on local data (for example .dev.vars and local KVs)

So it might be worth it to create a new how-to page dedicated to getCloudflareContext to exemplify some of these aspects

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Original conversation: opennextjs/opennextjs-cloudflare#330 (comment)

[vicb]

Done in #92